Lithography is a key factor in the drive for higher levels of micro-circuit integration. Dynamic RAMs have quadrupled in the level of integration every three years as a result of the reduction in minimum geometries and increases in chip size. As minimum geometries approach 0.5 .mu.m and below, lithography alternatives include optics, electron beam direct write, X-ray and electron/ion beam proximity technologies. The latter three technologies are still in their infancy relative to optical lithography and still have obstacles to overcome, including decreased throughput, low source brightness and mask complexity, respectively.
Optical lithography continues to be the dominant technology because it is well established and faces no formidable technical barriers to implementing sub-micron resolution at least as low as 0.35 .mu.m. The most common lithographic technique for the manufacture of integrated circuits on semiconductor wafers is a step-and-repeat procedure, i.e., a stepper, using reducing optics on the order of 5:1 or 10:1. Steppers offer level-to-level registration precision that is independent of wafer size by separate alignment of each exposure field. Further, the use of reducing optics makes mask production and handling more convenient. Steppers have a disadvantage, however, in that each stepper, although identically designed, is unique as a result of compensations in the complicated optics. As a result, it is often very difficult to align one level to previous levels if the previous levels were exposed using a different stepper.
Improvements in stepper resolution capability have resulted from the design and manufacture of high numerical aperture (N.A.) lenses and the move toward shorter wavelengths. While the majority of current steppers use a mercury lamp's G-line (436 nm) for exposure, the use of I-line (365 nm) exposure is becoming attractive for its shorter wavelength. Further, excimer lasers are being tested as light sources providing wavelengths in the deep UV region, including 248 nm (KrF), 193 nm (ArF) and 153 nm (F.sub.2). The limitation in implementation of the shorter wavelenqths has been the lenses due to the difficulty in manufacturing high N.A. lenses using deep UV transmissive materials. Most steppers use all refractive optics incorporating many optical elements, on the order of 10-20 lenses for an illumination system, all of which are made from optical quality quartz or fused silica which are subject to chromatic aberration. LiF and CaF.sub.2 are used for shorter wavelengths down to 248 nm. With broadband illumination systems, achromatic lenses can be used to correct chromatic variations but materials limitations for deep UV lenses make it extremely difficult to make a practical achromatic lens. For narrowband illumination which can attain deep UV, a chromatic lens may be used but the illumination must be restricted to a very narrow band and frequency stabilized. The narrowing of a source, usually a laser, increases the coherence effects such as formation of interference patterns and standing waves in the photoresist, can result in a significant reduction in irradiance and light uniformity at the wafer, and requires a more costly and technically complicated system.
In most types of refractive optics, lenses can be designed to correct optical aberrations such as spherical aberration, coma, astigmatism and field curvature. However, once a corrected lens is manufactured, its corrections are permanent and cannot be varied to compensate for additional environmentally-induced aberrations. The adaptive optic can be used to provide additional fixed optical aberration correction.
An alternative to refractive optics is an optical system which employs mirrors or a combination of mirrors and refractive optics as the primary imaging elements. The use of reflective optics provides the improvements of greatly reduced optical complexity, relatively broad chromatic correction and the ability to use any of a variety of available light sources including arc lamps and lasers, as long as the mirrors are appropriately coated for reflecting the desired wavelength.
The correction of aberrated wavefronts reflected from a mirrored surface and the addition of controlled distortions to laser signals is known and has particularly been used in ground-based telescopes where aberrations are caused, for example, by thermal gradients, atmospheric turbulence, etc. Selective local deformation of a mirror's reflecting surface may be achieved by the use of piezoelectric activators which are selectively energized by the application of electrical signals thereto to produce mechanical forces upon the rear surface of the mirror. Precise control of the distortions introduced into the mirror's reflecting surface may be achieved by spacing the actuators close to each other and by having the surface area of the mirror influenced by each actuator being kept as small as possible, and by making the structure which carries the reflecting surface as flexible as possible. Such a mirror may consist of a single thin sheet of reflective material as disclosed in U.S. Pat. No. 4,655,563 or may be a segmented mirror as disclosed in U.S. Pat. No. 4,944,580.
It would be desirable to provide an illumination system for a stepper which possesses the ability to compensate for optical aberrations with a deformable mirror as an element of either an entirely reflective system, a combination reflective/refractive optical system or as an add-on to a pure refractive system in which the compensation may be varied realtime as needed. It is to such a system that the present invention is directed.